In optical-fiber communication systems having multiple input and output ports, it is often advantageous to use a common interconnect bus and to employ multiplexing of communication signals at the various ports. This bus may comprise a bundle of optical fibers in parallel, each with its own surrounding cladding. For example, the cladding may consist of a concentric layer of a glass or epoxy resin having a slightly lower index of refraction than the signal-carrying silica fiber, thereby to confine the light rays by total internal reflection, as is well known. Alternatively, in some cases the fiber may be metallized to reflect the light rays.
Numerous configurations are possible for interconnecting a plurality of input ports with a plurality of output ports which may be greater, smaller, or equal in number. For greater reliability, it is common practice to transmit the same optical signals in parallel over an incoming bundle of fibers; then to mix and redistribute the signals substantially equally to the various output ports. Some output ports may be further subdivided so that one optical fiber may provide a plurality of low-level outputs used, for example, for optical feedback to control optical sources.
One known means for mixing a plurality of incoming optical signals from an incoming bundle of fibers and for redistributing them to another outgoing bundle of fibers is the transmission-type star coupler. Briefly, in known forms, the incoming fibers, each with associated cladding, are closely bundled together and their ends are terminated in a transverse plane. The light rays are then passed through a common optical mixing section, one end of which abuts the fiber ends and may be fused to them. From the mixing section, the light rays from all incoming fibers are then redistributed to another outgoing fiber bundle which abuts the other end of the star coupler and which may be similarly fused to it. The mixing section normally has a cross-section at least equal to the largest bundle cross-section and has its own outer potting casing, e.g., of epoxy resin. The mixing section may also have internal means for collimating light rays passing through it.
One serious limitation on the efficiency of such known star couplers is their considerable loss due to the packing fraction which is the ratio of total active core area to total bundle area in a plane transverse to the axis of the bundle. The packing fraction is usually substantially less than unity due to two factors:
(1) a packing factor due to stacking round fibers in a square or hexagonal close-pack array; and PA1 (2) a packing factor due to the relatively thick cladding layer around the core of each optical fiber.
Typical packing fractions for optical waveguide bundles are in the vicinity of 40%. Losses are usually greatest in bundles of single-mode optical fibers, as compared to multi-mode optical fibers, because their cladding is usually thicker. It has been proposed to increase the packing fraction by thinning down the fiber claddings within the bundle adjacent the mixing section, but in the past this has been at the expense of higher light attenuation and losses in optical transmission efficiency.